1. Field of the Invention
The present invention relates to a display device, which controls the operations of light emitting devices for display. More particularly, the invention relates to a technique available for light emitting devices such as organic light emitting diodes comprising a emissive layer having a reflective element provided on the rear surface thereof and a display device possessing such light emitting devices.
2. Description of the Related Arts
Organic light emitting diodes elements (devices) which emits a light by injecting holes and electrons into a emissive layer to thereby convert an electric energy into a light energy. Such types of display devices (hereinafter sometimes abbreviated as “OLED display devices”), which is emission type ones, have a characterized to have a thin type and a light weight unlike non-emissive type ones represented by liquid crystal devices. Furthermore, OLED display devices are characterized to have a wide viewing angle and have a rapid response time.
FIG. 22 is a schematic cross-sectional view showing one example of the conventional OLED display device. The OLED display device shown in this figure is composed of a transparent electrode 200 having a function of an anode, a hole transporting layer 102, an emissive layer 100, an electron transporting layer 101, and a reflective electrode 300 comprising a light reflective metal serving as a cathode deposited on a transparent substrate 400 in this order. When direct current voltage is applied between the transparent electrode 200 and the reflective electrode 300, the holes, which have been injected from the transparent electrode 200 arrive at the emissive layer 100 via the hole transporting layer 102 and electrons injected from the reflective electrode 300 arrive at the emissive layer 100 via the hole transporting layer 101, where the electrons and holes are recombined and the emission is brought about there-from.
Amongst lights emitted from the emissive layer 100, the lights 1000 directing towards the transparent electrode 200 are passed through the transparent electrode 200 and then are emitted from the transparent substrate 400. The lights 1001 directing towards the reflective electrode 300 are reflected at the reflective electrode 300, then are passed through the emissive layer 100, the transparent electrode 200 and the like, and are similarly emitted from the transparent substrate 400. Consequently, in such a type of OLED display device, it is important for obtaining a bright image to use an electrode having a high reflectance as the reflective electrode whereby the quantities of the lights emitted from the side of the transparent electrode is increased.
In such a configuration as described above, since the reflective electrode is in a state of mirror having a high reflectance when the OLED display device is in the state where it emits no light, under a bright environment, the image quality is deteriorated due to the fact that surrounding backgrounds are reflected in the reflective electrode and the image which should be displayed in black is not becomes dark, reducing a contrast ratio. These lead to problems, which should be solved. As one means for solving such problems, a configuration has been put into practical use in which a circular polarizer plate 800 is placed at the light emitting side of the transparent electrode 400. The circular polarizer plate 800 is composed of a polarizer plate 600 and a phase plate 700 serving as a quarter wave plate. The circular polarizer plate 800 is acted as follows:
An ambient light entering in the OLED display device from the circumference is an un-polarized light as a rule. Upon passing the ambient light through the polarizer plate 600, a linearly polarized light is transmitted through the polarizer plate 600, and a linearly polarized light perpendicular to the light just mentioned is absorbed thereon. The linearly polarized light having been transmitted through the polarizer plate 600 has an influence of the phase plate 700 to be circularly polarized light (in this case, for example, dextrorotatory circularly polarized light). Upon being reflected at the reflective electrode 300, the circularly polarized light having been passed through the phase plate 700 becomes a circularly polarized light whose helicity direction is reversed (levorotatory circularly polarized light). The light 2000R having been reflected at the reflective electrode 600 again enters in the phase plate 700, at which it has an influence of the phase plate 700 at the time of passing through the phase plate 700 to be converted into a linearly polarized light. In this case, the linearly polarized light having been converted is absorbed on the polarizer plate 600 and, thus, it is not returned to the external system. Specifically, the reflection of the ambient light on the reflective electrode 300 is reduced to darken the displaying of a black image, whereby the contrast ratio is remarkably improved. Such a construction is described, for example, in Japanese Patent Laid-Open Publication Nos. 8-509834 and 9-127885, which are incorporated herein by references. However, the OLED display device having a circular polarizer plate is disadvantageous in the fact that the displaying of the images are darkened since parts of lights emitting from the emissive layer are absorbed on the circular polarizer plate. This is due to the fact that since the lights emitting from the emissive layer are generally un-polarized lights and, thus, approximately half of the light is are absorbed on the polarizer plate making up the circular polarizer plate.
As a method for decreasing the lights absorbed on the polarizer plate to realize bright displaying, an OLED display device has been suggested, which has means for selectively reflecting circular polarized light comprising a cholesteric liquid crystal layer disposed between a quarter wave plate and a emissive layer. Such a construction is disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 2001-311826 and 2001-357979, which are incorporated herein by references. In this case, the lights emitting from the emissive layer enter in the cholesteric liquid crystal layer at which a specific circularly polarized light component is reflected, and a circularly polarized light component having a helicity direction different from that of the former is transmitted. When being passed through the quarter wave plate, the light having been transmitted through the cholesteric liquid crystal layer has an influence of the quarter wave plate to be converted into a linearly polarized light, which is transmitted through the polarizer plate.
On the other hand, the light reflected at the cholesteric liquid crystal layer is returned to the emissive layer and then reflected at the reflective electrode, at the time of this reflection, it becomes a circularly polarized light having a reverse helicity direction. The light reflected at the reflective electrode again enters in the cholesteric liquid crystal layer, at this time, it is passed there-through and has an influence of the quarter wave plate to thereby be converted into a linearly polarized light, which is transmitted through the polarizer plate. Specifically, amongst the lights emitting from the emissive layer, the lights which are polarized light to be absorbed on the polarizer plate are reflected at the cholesteric liquid crystal layer, before they are absorbed on the polarizer plate, whereby they are recycled. This obtains bright displaying of the images.
In the technique just mentioned, since lights which emit from the emissive layer and are transmitted through the polarizer plate, are increased, much more bright displaying of the image can be obtained in comparison with the OLED display device only having a circularly polarizer plate. However, in the case of using the later OLED display device under a bright ambient condition, there arises the following problems associated with ambient lights, which will enter in the later OLED display device: The ambient lights entering in the OLED display device are generally un-polarized lights and at least halves of them are adsorbed on the polarizer plate, when they are passed through the polarizer plate. When being transmitted through the quarter wave plate, the lights having been passed through the quarter wave plate have an influence thereof to be circularly polarized lights (for example, dextrorotatory circularly polarized light), and is transmitted through the cholesteric liquid crystal layer. Upon transmitting the lights having been passed through the cholesteric liquid crystal layer through the emissive layer while substantially maintaining their polarized states, and at the time of the reflection at the reflective electrode, they becomes circularly polarized lights whose helicity direction is reversed (levorotatory circularly polarized lights), and then reflected again when entering in the cholesteric liquid crystal layer.
Since the lights reflected at the cholesteric liquid crystal layer again reflected at the reflective electrode to be a circularly polarized light having a reverse helicity direction (dextrorotatory circularly polarized light), the light at this time are transmitted through the cholesteric liquid crystal layer, passed through the quarter wave plate and the polarizer plate, whereby they exit out of the OLED display device. This means that an unnecessary reflection of the ambient light is increased by the arrangement of the cholesteric liquid crystal layer and, thus, indicates that the black image cannot be displayed in a sufficient manner under a bright condition, leading to markedly decreasing of the contrast ratio.
According to these prior arts described above, there is a description that in order to realize a wide wavelength range of selective reflection within the visible wavelength range, a plurality of cholesteric liquid crystal layers each having a different helical pitch are deposited. As one embodiment of the prior art, the central wavelength of the selective reflection at the cholesteric liquid crystal layers is set to be 550 nm, which is a high relative luminous efficiency in a photopic vision. These conditions are the conditions where the unnecessary reflection of the ambient light brought about by placing the cholesteric liquid crystal layers becomes large, and thus, lead to a remarkable decrease in the contrast ratio under a bright condition. Specifically, in the prior art, there is no description for the problem for increasing the reflection of the ambient light, which occurs in the case of the display device having the polarization separator such as the cholesteric liquid crystal layers, and no deal has been made.
As one method for realizing a full color display device using an organic light-emitting diode, a method in which pixels corresponding to three primary colors (red (R), green (G), and blue (B)) are directly patterned has been suggested. This method can be expected to realize a high efficiency by forming the pixels for respective colors under the optimum conditions. However, since the existing organic light-emitting diodes have the wavelength of the light emission deviating from the desirable wavelength or since the distribution of the wavelength for light emission is wide and gentle, no sufficient color reproduction can be obtained.
Also, since the luminous efficiency (lm/W) is differed in the colors, the power consumption for displaying white becomes large. At the present situation, the organic light-emitting diode for green light emission has the highest luminous efficiency, but since the balance of chromaticity of each color is bad, it is required that the luminous intensity of the organic light-emitting diode for green light emission, which has a high luminous efficiency is relatively decreased, and the luminous intensities of the organic light-emitting diodes for red and blue light emission are increased, leading to decreased total efficiency.
The present invention has been done in light of the above situation, and an object of the present invention is to provide a display device which can realize bright display by effectively contributing the light emitted from the organic light-emitting diode to display, and which can realize display with a high contrast even under a bright condition by decreasing the reflection of the ambient light. Also, an object of the present invention is to provide a color display device, which shortens the difference of the power in colors and enhances the efficiency. Another objects will be apparent from the following description.